Do you have them on any censuses?
By 1911 I think they may be indexed as "TREADNELL" and Mary Ann listed as a widow.
The usual reasons you might not be able to find a marriage:
a) different names used (mother was married before, first names/middle names swapped around, different spellings, aliases, etc).
b) marriage took place outside England/Wales
c) marriage took place much later than expected (well after birth of the children, for example)
d) marriage didn't take place.